PHYSICAL SIMULATION OF CONTINUOUS ROLL CASTING PROCESS

ACTA METALLURGICA SINICA (ENGLISH LETTERS)Vol. 18 No. 4 pp 505-511 August 2005PHYSICAL SIMULATION OF CONTINUOUS ROLLCASTING PROCESSL.H. Zhan, J. Zhong, X.Q. Li and M.H. HuangCollege of Electronical-Mechanical Engineering, Central South University, Changsha 410083,ChinaManuscript received 9 June 2004; in revised form 17 March 2005A series of simulating experimental studies on the rheological behavior and its influential factorsof aluminum alloy in continuous roll-casting process have been explored in this paper with aGleeble-1500 Thermal-Mechanical Simulation Tester and a set of special clamp system. Relevantrheological rules in the process of coupling transient solidification and continuous deformationof rll-casting conditions are obtained. Experimental results indicate that four diferent charac-teristic stages exist in the whole rheological process, and relative constitutive models suitable forthe given conditions of continuous roll casting process have been established through multivari-able linear regression analysis of the experimental data.KEY WORDScontinuous roll-casting, rheological behavior, physical simulation,constitutive relation1. IntroductionWhen carrying out force and power calculation in hot working, average strain rate and temperatureare usually used. Urcola and Sellarl studied the effect of changing strain rate on the strain-stress relationof material on high temperature condition. They found that the flow behavior of commercial pure alu-minum was related to the history of strain rate while ferritic stainless steel and Al-1%Mg alloy were not.The strain rate and temperature continuously change in the roll-casting process. Especially the changingof temperature, great temperature gradient exists in the roll- casting zone and the average value reachesabove 200°C /cm2! Such kind of intensive temperature change has great influence on deformation resis-tance of material.Researches on rheological behavior of continuous roll casting process now are usually based on thehydrokinetics and plastic mechanics，and take rheological behavior of material in roll-casting zone asNewton fluid flow and rigid plastic deformation for the liquid and solid zone metal separately, which cannot exactly describe its true evolution. Hitherto, researches on the dynamic strain-stress relation are verydeficiency, which influenced the development of roll-casting technology.Dr. Liu et al.B 41 carried out physical simulation of continuous roll-casting process in Gleeble- 1500thermal-mechanical simulation tester, and similar microstr|中国煤化工asting process havebeen achieved. A set of special clamp system is developeMHCNMHG3- 1500 thermal-me-chanical simulation tester to simulate the continuous roll casung process In un1s paper, and a series of ex-perimental researches on the rheological behavior of the material and jits influential factors have been506done，the constitutive relations of complex rheological behavior in continuous roll-casting process areobtained.2. Physical Simulation of Roll-Casting Process2.1 Experimental equipmentA Gleeble- 1500 thermal-mechanical simulation tester has been used in the experiment. In order tosimulate the actual roll-casting conditions, a set of special clamp system has been developed (as shownin Fig. la), the clamp is made of copper and the foundation is of steel. And a peculiar water-cooling sys-tem has been designed to get strong cooling intensity in the clamp system.2.2 Experimental principle and procedureThe dimension of testing specimen is machined to 10mm in diameter and 15mm in height. Differ-encial heat analysis shows that the melting temperature zone of experimental used aluminum alloy is 643and 655°C for solidus and liquidus, separately. Main experimental processes are as follows:(1) In order to get the stable deformation process under continuous loading, some red copper foilhas been used in the experiment to wrap around the specimens, which can guarantee the synchronousmovement of specimen with the applied load.(2) In order to simulate the transient solidification and synchronous rolling conditions of the actualroll-casting process, the specimens being wrapped around with red copper foil are firstly put into themuffle to heat to the melting state, and keep such state for a period of time, then swiftly put them into themidst of the experimental table of the special clamp system to carry out quickly cooling and compressionoperation as the experiment set conditions, ie. the strain rate and the strain value.(3) The temperature variation can be achieved by the thermocouple inserted into the centre of thespecimen surface. The strain and strain rate can be rigorously directed with the control system of Glee-ble- 1500 thermal-mechanical simulation tester and the results are collected in real-time.The strain rate used in this experiment is over a wide range of 0.01-10.0s;the strain is less than orequal to 0.8; the specimen are firstly heat to the temperature of 685"C, and keep for 5min, then put intothe experiment table when the temperature is larger than or equal to 655C, that is the liquidus tempera-ture, to start instantancous cooling and synchronous compression. The schematic graph of the simulatingequipment is shown in Fig.lb.InallSealed loopStemThin ceramic pipeThermocouple←ClampWater clling channelRed copper folMeting metalFoundation ottall中国煤化工a)MHCNMHGFig. I Schematic graph of simulation of continuous roll casting system: (a) clamp equipment;(b) simulating equipment.507.3. Physical Comparability AnalysisThe whole rheological course of instantaneous solidification and synchronous rolling hasbeen fulfilled in such modified equipment and physical comparability of the experiment to theactual continuous roll-casting process is as follows:(1) Both possess the similar solid-liquid phase transformation process and comparative temperaturechanging zone, the temperature-changing course of modeling experiment is basically consistent with theactual roll-casting process and the load was applied throughout the experimental process.(2) The strain rates and strain values used in simulating experiments are comparable with that in theactual roll casting processes (normal and/or transnormal roll-casting process). .4. Experimental Results and Discussion4.1 Relationship of flow stress, temperature and strain under different strain rate conditionsThree-dimensional graphs, which describe the relation of flow stress, temperature and strain valuewhile the metal is cooled and synchronously compressed from melting state to final deformation temper-ature under different strain rate conditions, are obtained as shown in Fig.2.4.2 Experimental results analyses4.2.1 Effect of strain value ε on flow stress σIn order to consider the effect of strain value on the flow stress, experiments are carried out at con-stant strain rates; through changing the cooling ability of the clamp system to make the melting metal un-dergo different rhelolgical courses of instantaneous solidification and synchronous rolling, that is to say,at the same temperature, the strain values are different. Fig.3 shows the relation between the flow stressand strain value.Fig.3 indicates that the strain value has slight influence on the flow stress than the effect of tempera-ture under the conditions of simulating roll casting experiment. For example, when the strain rate 8 is .0.5s"，and the defornation temperature T is 450°C，the flow stress σ obtained from Fig.4 is 45.0 and46.5MPa related to the strain value ε is 0.55 and 0.628, respectively. While when the deformation tem-perature is changed to 400C, the flow stress is 65.7MPa related to the strain value is 0.59.a)b)1407020(60号10c。50星.8o0; 4(eo4020.0 0.1 0.2 0.3 04 0.5 0.6 0.7 0.8中国煤化工7StrainMHCNMHGFig.2 Relationship of flow stress, temperature and strain at different strain rates:(a)&=0.1s*;(b) 8=0.5s-+.508700间)0.eb) 0.6600P-S(2)650P-S(2- 0.。500-P-S(1)+0..4400上-0.300 tP-T(2)| 0.2200 H0.1450P-T(1)100F0.0400.040 680 100 120 1400102030405060 7080Pressure, MPaFig.3 Relation of flow stress and strain value at the same strain rate and temperature condi-tions: (a)&=0.1s";(b) e=0.Ss- . Here P-S(1) and P-S(2) show the curves of flow stressus. strain value, P-T(1 )and P-T(2) show the curves of flow stress us. temperature.Fig.4 shows the variation of flow stressoFwith strain value when strain rate is0.1s'. Ito300'c.indicates that if the temperature is higher than450C, with the increase of strain value, flow号50-350°C- + -stress increases a lttle; while if the tempera-40-400°C- +ture is equal to or less than 400C, flow stress230-450'C，is independent of the strain value. So it is ap-500°c_550'Cpropriate to neglect the effect of strain valueon flow stress under the condition of simulat-0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55Straining roll-casting experiments.4.2.2 Effect of temperature T on flow stress σFig.4 Variational curve of flow stress to strain valueIt is seen from Fig.5 that the tendencyin the simulating roll-casting expcriment.the flow stress changes with the temperatureis basically the same for different strain rate, and with the decreases of temperature, the flow stress in-creases.4.2.3 Effect of strain rate 8 on flow stress σThe regularity that flow stress changes with the variation of strain rate has been found through ex-periments, as shown in Fig.6. Fig.6 shows that, with the increase of strain rate, flow stress increase, but itdoesn't in a linearity correlation.4.3 Rheological constitutive model of continuous roll-castingMost researchers simplified constitutive behavior of the material in continuous roll casting process,usually follow the formulae of hot rolling process or some empirical ones50. Such formulae can't com-pletely describe the actual conditions in roll casting process and have great limits.In the actual plastic working, flow stress of the materi:中国煤化工iexertedonitandthe energy it dissipates. Except for the deformation temperatMHC N M H Gd strain value (8),the composition, grain size, the heat treatment institution and the deformation history, etc., all can influ-ence the flow stress valuel", which can be expressed as50916012> 300°C1401003350°C80400C， 8Cg6450°C曾40500'c4050"C[20600"C100 20300500600000.2O..81.0Temperature, °cStrain rate, s'Fig.5 Relation of flow stress and temperature (1.8=0.0ls+;Fig.6 Regularity of flow stress changes with strain rate.2.8=0.1s';3.8=0.5s;4. =1.0s').σ=f(e, e, T,C)(1)where C represents the other conditions except for the strain rate (E), deformation temperature (T) andstrain value (8).As the material and its initial microstructure is usually same in the actual deformation conditions, Ccan be taken as a constant, and its influence can be neglected, then Eq.(1) can be rewritten asσ=f(e, ε,7)(2)Existing formulae used to express the relation between flow stress and strain rate in high tempera-ture are as follows(I) Exponential relation, which is mainly suitable for high applied stressts&x exp (βr)(3)(I) Power law relation, mainly used to low applied stressε∞σ”(4)(II) Hyperbolic sine relation, which is applicable in a wide range of strain rate and flow stress& x [sinh(aσ)]"(5)When aor<0.8, Eq.(5) reduces to power law relation; while when 0r>1.2 , this equation becomes expo-nential relation'", whereB, n, a, n' are temperature independent constants, and c= B/n.4.4 Establishment of rheological constitutive models for continuous roll casting processSynthetic all the above analysis, divided the complex rheological process of continuous roll- castingof aluminum into four different stages. Through multivariate linear regression analysis of the experimen-tal data，the constitutive equations which applicable for the special conditions of continuous roll-astingprocess are set up, that is:中国煤化工(I) When the temperature is higher than 600°C, theMHC N M H Ged as creep deforma-tion, material possesses Newtonian viscosity, deformation is independent of dislocation while only re-lates to the diffusion process510σ=2.37x10*ε08733 exp(-0.019T)(6)(I) When the temperature lies in the range of 500-600C, and for lower strain rate, deformation canbe regarded as high temperature and low stress level deformationσ=3.71x10':06707 exp(-0.01717)(I) When the temperature falls in the range of 300- 500°C , and for lower strain rate, the deforma-tion can be considered as middle stress level deformationσ=9.161x10[sinh-(E 04)] exp(-0.00697)(8)(IV) When the temperature is lower than 300C , and the strain rate is less than 1.0s", deformationbelongs to middle stress level deformation; while when the strain rate is larger than 1.0s', the deforma-tion lies in the high stress level deformation stageσ=2.356x103g05345 exp(-0.0041T)(9)Comparing of the results calculated by using the regression model with the experimental ones atdifferent strain rates are given in Fig.7. Fig.7 indicates that the results calculated by using the regressionmodel fit well with the experimental ones, which gives support to the validity of the regression model.140(a120■Calculated resuts(b- Experimental data10080出40车40-200一o叶00400o60050300350400 450500550600 650700Temperature, °CFig.7 Comparing of calculated results with experimental ones at different strain rates:(a)&=0.1s*;(b) s=0.5s-*.5. Conclusions(1) Physical simulating of the roll-casting process has been realized with the experimental frock andtesting system modified in the Gleeble- I 500 thermal-mechanical simulation tester.(2) There are four different characteristic stages during中国煤化工process of continu-ous roll-casting, and different constitutive models related toMHCNMHGsetup.(3) Results calculated from established constitutive models tally well with experimental data, whichproves the reliability of such models.511.Acknowledgements- This work was supported by the National High Technical Reasearch and Development Pro-gramme of China (No. 19990604906).REFERENCES1 J. Urcola and C.M. Sellars, Acta Metall. 35(1) (1987) 2637.2 X.Q. Li and X.L. Huang, Journal of Central South Uniwersily of Technology 29(4) (1998) 374 (in Chinese).3 Y. Liu, K.C. Zhou, Y.H. Lin and H.B. Wang, The Chinese Journal of Nonferrous Metals 13(3) (2003) 589 (inChinese).4 Y.Liu, Y.H. Lin and K.C. 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